cgpa_calculator

defmodule CgpaCalculator do
  @moduledoc """
  A lightweight Elixir library for calculating GPA, CGPA, and SGPA.
  Supports 4.0 and 5.0 grading scales.

  Homepage: https://gpacalc.app
  """

  @grade_points_4 %{
    "A+" => 4.0, "A" => 4.0, "A-" => 3.7,
    "B+" => 3.3, "B" => 3.0, "B-" => 2.7,
    "C+" => 2.3, "C" => 2.0, "C-" => 1.7,
    "D+" => 1.3, "D" => 1.0, "D-" => 0.7,
    "F" => 0.0
  }

  @grade_points_5 %{
    "A" => 5.0, "B" => 4.0, "C" => 3.0,
    "D" => 2.0, "E" => 1.0, "F" => 0.0
  }

  @doc """
  Calculate GPA for a list of courses.

  ## Examples

      iex> CgpaCalculator.calculate_gpa([%{grade: "A", credits: 3}, %{grade: "B", credits: 3}])
      3.5

  """
  def calculate_gpa(courses, scale \\ 4.0) do
    grade_map = if scale == 5.0, do: @grade_points_5, else: @grade_points_4

    {total_points, total_credits} =
      Enum.reduce(courses, {0.0, 0.0}, fn course, {pts, creds} ->
        grade = String.upcase(to_string(course.grade))
        credits = course.credits / 1
        point = Map.fetch!(grade_map, grade)
        {pts + point * credits, creds + credits}
      end)

    Float.round(total_points / total_credits, 2)
  end

  @doc """
  Calculate cumulative GPA across semesters.

  ## Examples

      iex> CgpaCalculator.calculate_cgpa([%{gpa: 3.5, credits: 15}, %{gpa: 3.8, credits: 16}])
      3.66

  """
  def calculate_cgpa(semesters) do
    {total_points, total_credits} =
      Enum.reduce(semesters, {0.0, 0.0}, fn sem, {pts, creds} ->
        {pts + sem.gpa * sem.credits, creds + sem.credits}
      end)

    Float.round(total_points / total_credits, 2)
  end

  @doc """
  Convert percentage to GPA on a 4.0 scale.
  """
  def percentage_to_gpa(pct) when pct >= 93, do: 4.0
  def percentage_to_gpa(pct) when pct >= 90, do: 3.7
  def percentage_to_gpa(pct) when pct >= 87, do: 3.3
  def percentage_to_gpa(pct) when pct >= 83, do: 3.0
  def percentage_to_gpa(pct) when pct >= 80, do: 2.7
  def percentage_to_gpa(pct) when pct >= 77, do: 2.3
  def percentage_to_gpa(pct) when pct >= 73, do: 2.0
  def percentage_to_gpa(pct) when pct >= 70, do: 1.7
  def percentage_to_gpa(pct) when pct >= 67, do: 1.3
  def percentage_to_gpa(pct) when pct >= 63, do: 1.0
  def percentage_to_gpa(pct) when pct >= 60, do: 0.7
  def percentage_to_gpa(_), do: 0.0

  @doc """
  Convert GPA to letter grade.
  """
  def gpa_to_letter(gpa) when gpa >= 3.85, do: "A"
  def gpa_to_letter(gpa) when gpa >= 3.5, do: "A-"
  def gpa_to_letter(gpa) when gpa >= 3.15, do: "B+"
  def gpa_to_letter(gpa) when gpa >= 2.85, do: "B"
  def gpa_to_letter(gpa) when gpa >= 2.5, do: "B-"
  def gpa_to_letter(gpa) when gpa >= 2.15, do: "C+"
  def gpa_to_letter(gpa) when gpa >= 1.85, do: "C"
  def gpa_to_letter(gpa) when gpa >= 1.5, do: "C-"
  def gpa_to_letter(gpa) when gpa >= 1.15, do: "D+"
  def gpa_to_letter(gpa) when gpa >= 0.85, do: "D"
  def gpa_to_letter(gpa) when gpa >= 0.5, do: "D-"
  def gpa_to_letter(_), do: "F"
end